RF Repeater Isolation Explaination

RF REPEATER ISOLATION EXPLAINATION Isolation means the attenuation between service and donor port of Repeater, is very important for RF Repeater system, especially for outdoor repeater system. If the isolation is not enough, the system will oscillate itself and then interfere the BTS. when we survey a RF Repeater site, we have to consider the Isolation, we can make Isolation in horizontal plane or vertical plane. In theory, we can estimate the Isolation according to the formula as below. In horizontal plane:Lh=22.0+20log10(d/λ)-(Gt+Gr)+(Xt+Xr) (1) thereinto： “22.0” is transmission constant “d” is distance between service antenna and donor antenna “λ” is wave length Gt、Gr are Gain of service antenna and donor antenna Xt、Xr are F/B of service antenna and donor antenna In vertical plane：Lv＝28.0+40log10(d/λ) (2) thereinto： “28.0” is transmission constant “d” is distance between service antenna and donor antenna “λ” is wave length There is a list of Isolation value corresponding with the distance in vertical plane as reference

850MHz Distance(m) Isolation(dB) 1900MHz Distance(m) Isolation(dB) 2100MHz Distance(m) Isolation(dB)

10 86

12 89

15 93

18 96

20 98

25 102

10 100

12 103

15 107

18 110

20 112

25 115

10 101

12 104

15 108

18 112

20 113

25 117

There is a list of Isolation value corresponding with the distance in horizontal plane as reference

850MHz Distance(m) Isolation(dB) 1900MHz Distance(m) Isolation(dB) 2100MHz Distance(m) Isolation(dB)

10 81

15 84

20 87

25 89

30 90

35 91

10 88

15 91

20 94

25 95

30 97

35 98

10 88

15 92

20 94

25 96

30 98

35 99

All RF repeaters must agree with following formula to ensure the system stable: Po=Pin+Gworking, Isolation > Gworking+10~15 dB, Obviously, if the input level is low, the working Gain must be bigger to achieve the high output power, so that the isolation must be bigger accordingly, in consequence we have to make bigger separation between donor antenna and service antenna; but in the other hand, if we adopt high gain donor antenna(parabolic antenna),so we can increase input signal level, so that we can lower the working gain to achieve the output power, therefore the isolation requirement is easier to achieved. For example, if we design output power of Repeater 40dBm, and the input power is –45dBm (this level can be reached under the condition: less than 10Km and LOS between donor antenna with 15dB gain and BTS antenna),we need to set Repeater Gain at 85db. The Repeater must work at the Gain 10dB less than the system Isolation. Therefore we need 95dB isolation in this case. In other word, we need about 18 m distance in vertical plane. In addition we can choose higher gain donor antenna to improve receiving signal level in some difficult site. In engineering, we also can take advantage of the barrier (wall or trees) or install an electromagnetic shield to improve the system Isolation. If all engineering measures couldn’t help, we can change to ICS repeater to solve this. Anyway we must consider the input, gain, output and isolation comprehensively to ensure the repeater system working stable, not only output power while commissioning.